The present invention relates to beam alignment systems, and more particularly to a beam alignment system that may be used in an optical disk storage system to accurately set and maintain data track pitch.
In an optical disk storage system, data is stored by marking a rotating disk with a beam of radiant energy (typically a laser beam) that is modulated in some fashion by the data to be stored. To write or store data on the disk, the modulated beam is directed to and focused at a desired point (termed the "write point" for purposes of this application) on the surface of the disk. As the disk rotates under the write point, a "data track" is created by the marks made on the disk by the modulated beam. If the write point is held stationary, a circular data track is created centered about the axis of rotation of the rotating disk. Additional data tracks, each concentric with the others, can be created by blanking the write beam off, moving the write point radially with respect to the disk to a new location, holding the write point stationary at this new location, and turning the modulated write beam back on. Alternatively, if the write point is radially moved with respect to the disk as the modulated write beam makes marks thereon, a spiralling data track is created on the surface of the disk.
Whether the data tracks are concentric or spiralling, the available surface area on the disk is most efficiently used when the data tracks are spaced together as close as possible. The radial distance between adjacent data tracks is called the "track pitch". Accurately maintaining the track pitch at a desired value, especially where the track pitch must be kept small so as to efficiently make use of the storage space available on the disk, has presented a significant obstacle in the development of high storage capacity optical disk storage systems.
To read data that has been previously stored or written on the disk, a read beam of radiant energy is directed to a desired data track on the disk. This read beam typically has different parameters associated therewith than does the write beam (such as intensity and/or wavelength), thereby ensuring that the read beam does not mark the disk in the same manner as the write beam is designed to mark the disk. The read beam is either reflected from the surface of the disk, or passes therethrough (if the substrate of the optical disk is sufficiently transparent to allow the beam to pass therethrough), and the intensity of the read beam is modulated in accordance with the data marks that have been previously written in the data track by a write beam. The data marks typically comprise a sequence of reflectivity-high/reflectivity-low (or transmissivity-high/transmissivity-low) marks that modulate the reflected or transmitted read beam in accordance with the pattern of the stored data. Once modulated, the read beam is directed to a suitable optical detector where a modulated data signal is generated. The data is extracted from this signal using conventional demodulation techniques.
Schemes are known in the art (see U.S. Pat. No. 3,876,842) for using a plurality of read beams to read and track a data track written on a flat record carrier, such as an optical disk. It is also known in the art to place a new data track at a "constant distance from the next preceding already recorded data track so that the data tracks are maintained in spaced parallel alignment without crossing each other," U.S. Pat. No. 3,657,707, col. 11, lines 40-43. However, the particular scheme shown in U.S. Pat. No. 3,657,707 for achieving the desired spaced parallel alignment falls short of providing and maintaining an accurate and narrow track pitch of the type required in high data capacity optical storage systems.